By way of background, solar receivers have, in the past, been mounted on towers adjacent a mirror field which redirects solar radiation and focuses it onto the solar receiver. As illustrated in co-pending application Ser. No. 612,434 of Philip O. Jarvinen assigned to the assignee hereof, it is possible to devise a closed cavity solar receiver in which the cavity carries a honeycomb heat exchanger and has a window positioned in the aperture of the cavity. Focused solar radiation passes through the window and impinges on the honeycomb heat exchanger at which time air flowing through the honeycomb is heated.
It is possible to eliminate the window and operate the receiver at ambient pressure, in which the air moving through the receiver is at 1 atmosphere. This type system is especially advantageous in large scale receivers capable, for instance, of collecting enough energy to power a 100 megawatt electric generator since large windows and support structure for the windows need not be provided.
One of the problems with operating an open cavity receiver is that the receiver is normally subjected to transient wind conditions which may cause an exchange of the receiver air with ambient resulting in losses of thermal energy from within the receiver through the cavity opening.
Absent airflow at the aperture of the receiver, the exchange between the heated air within the receiver and ambient is one of diffusion which operates relatively slowly and therefore results in only negligible heat loss. With airflow at the aperture of the solar receiver there is an order of magnitude increase in the exchange. This is primarily due to turbulent-flow exchange which involves the entraining of the quiescent gases at the aperture in a jet, which in the present case, is formed by the edge of the wind which dips toward the interface between the hot air in the receiver and the ambient.
This exchange can result in substantial losses which, in the present invention, are prevented by providing means for deflecting the "critical" streamline away from the receiver aperture. It will be appreciated that the further the streamline is deflected away from the aperture, the less will be the exchange. The "critical" streamline is the one closest to the receiver aperture and it is this streamline which is deflected from its natural position by a deflector such as an air foil or an active jet stream. Either one of these devices gives the airflow in the vicinity of the aperture momentum in a direction away from the aperture. As such, the present invention includes either a passive device with a specialized aerodynamic structure or active means for deflecting the wind away from the open end of the receiver so as to provide a dead air zone. As will be seen, the wind deflecting apparatus may be utilized with receivers which have a so-called "terminal concentrator" which concentrates focused solar radiation at the receiver aperture, or in situations in which no terminal concentration is utilized. When terminal concentrators are used, the deflection apparatus may be located at the lip of the concentrator. Otherwise, the deflection apparatus is located at the aperture of the receiver.
It should be noted that wind, in general, comes in horizontally. When terminal concentrators are used, the reattachment point for the flow stream occurs to the leeward side of the terminal concentrator away from the receiver aperture, if a deflection foil is used. In essence, wind impinging on the deflection foil produces a flow stream which displaces wind away from the aperture of the receiver thereby to form a dead air zone at the open end of the receiver. The only circulation at the receiver aperture is that due to low energy vortices which do not materially affect the operation of the receiver.
The active system includes in one embodiment the formation of outwardly projecting air jets at the lip of the terminal concentrator. This is simply accomplished by a channeled or perforated ring at the concentrator lip. These jets may be produced annularly or may only occupy as little as one-third the periphery of the ring. Pressurized air within the ring forms jets which project outwardly and deflect wind completely away from the receiver aperture.
It will be appreciated that solar energy receivers mounted in a central power tower in general face downwardly. However, for the most part, they are not vertically oriented but rather are tilted off the vertical axis by as much as 14.degree.. Without the protection offered by the subject system, for positive angles of attack, the wind reattaches within or very close to the receiver aperture and large heat losses are experienced. Even for negative angles of attack the flow stream reattachment point may be sufficiently close to the open aperture to cause thermal loss.
With a tilted receiver as defined above, providing either the passive deflection foil or the active jet system, prohibits wind having either a positive or a negative angle of attack from affecting the operation of the receiver. The positive and negative angles of attack refer to the fore and aft direction with respect to the receiver orientation. It will be appreciated that wind coming in sidewise, in essence, has a zero angle of attack. However, with a zero angle of attack, thermal losses also occur, and it is advantageous to have either the passive or active wind deflection system for side gusts so that receiver thermal losses are held to acceptable levels.
It is, therefore, an object of this invention to provide an open cavity solar receiver with adequate protection against ambient wind conditions which would affect the operation of the receiver;
It is another object of this invention to provide either active or passive heat loss protection by providing the solar receiver with annular means for deflecting wind away from the aperture in the receiver cavity;
It is a still further object of this invention to provide an open cavity solar energy receiver with a heat exchanger in the cavity and means at the aperture of the cavity for providing a dead air zone thereat, thereby to prevent losses due to ambient wind conditions;
These and other objects of this invention will be better understood in connection with the following specification taken in conjunction with the appended drawings in which